This present invention relates to the field of anechoic chambers, and in particular to a modular anechoic panel system and method.
The character and quality of noise emitted from manufactured products has become increasingly important to the function and marketability of such manufactured products. Product manufacturers, governments, and standard setting organizations often require consumer and industrial products and equipment to comply with increasingly stringent sound emission specification. Accordingly, a large number of consumer products and industrial equipment must now undergo sound emission testing.
Anechoic chambers, constructed using acoustical anechoic wedges are frequently employed in such sound emissions tests. According to previous techniques, an anechoic chamber consists of a shell constructed of material to provide structural stability and predictable transmission loss characteristics from the exterior of the anechoic chamber to the interior of the anechoic chamber and an array of sound-absorbing anechoic wedge devices (xe2x80x9canechoic wedgesxe2x80x9d) lining the shell""s interior surfaces to eliminate interior reflected sound. Materials used in the construction of shells for anechoic chambers have included various materials, such as masonry, wood, and metal. Shell designs have included permanent shell structures, as well as semi-permanent shells constructed of modular interlocking structural panels. Anechoic chambers with anechoic wedges or other linings on all interior surfaces are typically referred to as xe2x80x9cfullxe2x80x9d anechoic chambers while chambers having linings on only the walls and ceiling are referred to as xe2x80x9chemixe2x80x9d anechoic chambers. Anechoic chambers, both hemi and full, are used in the testing and or measurement of sound characteristics emitted by a specimen being tested or calibrated.
An anechoic chamber is a room that is used for precise acoustical measurements. Therefore, the room must be designed so that acoustically free field conditions exist. For practical measurements, the room also must be free of extraneous noise interferences. Anechoic chambers are widely used in the development of quieter products, including automotive and aircraft products and other products for use in transportation, communications, computers, security, and medical research.
An acoustical free field exists in a homogeneous, isotropic medium that is free of reflecting boundaries. In an ideal free field environment, the inverse square law would function perfectly, so that the sound pressure level generated by a spherically radiating sound source decreases approximately six decibels (6 dB) for each doubling of the distance from the source. A room or enclosure designed and constructed to provide such an environment is called an anechoic chamber.
Also usually an anechoic chamber must provide an environment with controlled sound pressure (Lp) free from excessive variations in temperature, pressure and humidity. Outdoors, local variations in these conditions, as well as wind and reflections from the ground, can significantly and unpredictably disturb the uniform radiation of sound waves. This means that a true acoustical free field is only likely to be encountered inside an anechoic chamber. For an ideal free field to exist with perfect inverse square law characteristics, the boundaries must have a sound absorption coefficient of unity at all angles of incidence.
Anechoic chambers are characterized by anechoic elements that are attached to the walls, ceiling and floor of the chamber. If the anechoic elements are attached to the walls and ceiling but not the floor of the chamber, the chamber is termed a hemi-anechoic chamber. Such chambers also are used for acoustical measurements. The anechoic elements may be attached so that they are essentially in contact with or spaced from the supporting walls, ceiling and floor, depending on what is considered to be the optimum design for the chamber based on its intended use.
An anechoic element is commonly defined as one that should have less than a 0.99 normal incidence sound absorption coefficient through the frequency range of interest. In such case, the lowest frequency in a continuously decreasing frequency sweep at which the sound absorption coefficient is 0.99 at normal incidence is defined as the cut-off frequency. Thus, in an anechoic chamber, 99% of the sound at or above the cut-off frequency is absorbed. For less than ideal conditions, different absorption coefficients may be established to define a cut-off frequency. Heretofore anechoic elements for anechoic chambers have commonly been designed in the shape of a wedge.
As already noted, a characteristic of a true free field is that the sound behaves in accordance with the inverse square law. In the manufacture of anechoic elements, those elements are tested in impedance tubes as a means for qualifying them for use in chambers simulating free field conditions. A fully anechoic chamber can also be defined as one whose deviations fall within a maximum of about 1-1.15 dB from the inverse square law characteristics, depending on frequency. According to currently accepted standards, semi-anechoic rooms or chambers, i.e., those with anechioic walls and ceilings but with acoustically reflective floors, e.g., floors made of concrete, asphalt, steel, or other metals or materials, can deviate from the inverse square law by a maximum of about 3 dB depending on frequency.
Because of the very high degree of sound absorption required in an anechoic chamber, conventional anechoic elements typically comprise sound absorptive material covered or contained by a cage or cover that is made of a wire cloth (mesh) or a perforated sheet metal. For many years anechoic elements typically embodied a wire mesh cage that typically was characterized by a 90-95% open area to allow maximum exposure of sound absorbing material to the sound waves.
A disadvantage with anechoic construction elements as explained above is that in highly industrial environments the wire mesh structure may not provide sufficient physical protection for the elements. The sound absorbing material can therefore become easily disfigured by unintentional impact that is quite foreseeable in a heavily industrial environment.
Another disadvantage of the conventional anechoic elements is potential medical hazards. The sound absorptive materials such as fiberglass, rockwool or foams can be highly erosive. Over a period of use such materials could erode into particulate matter floating in the air which could be inhaled into lungs.
A further disadvantage of the conventional anechoic elements and their wire mesh coverings is that in highly industrial applications, oil spills and dirt may rapidly accumulate on the sound absorbing materials. This may impede sound absorption performance of the material and additionally may impose a fire hazard. Cleaning the sound absorptive material is difficult and not efficient.
More recently, the wire mesh covering has been replaced by a perforated sheet metal, with the an open area provided by the perforations falling within a relatively wide range: usually the open area falls within the range of about 23% to about 52% of the entire area of the sheet metal covering.
The earliest practical design for sound absorbing units of the type used in making anechoic chambers was a wedge-shaped unit fabricated from or comprising fibrous glass. That geometry of anechoic wedges has been employed as the basis for anechoic chamber design and construction in the past. Examples of prior art anechoic elements and chambers made using such elements are provided by U.S. Pat. Nos. 2,980,198, 3,421,273, and 5,317,113, and the technical publications by L. L. Beranek et al, xe2x80x9cThe Designs And Construction of Anechoic Sound Chambersxe2x80x9d, J. Acous. Soc. of America, Vol. 18, No. 1, pp.140-150, July 1946; and B. G. Watters, xe2x80x9cDesign of Wedges For Anechoic Chambersxe2x80x9d, Noise Control, pp. 368-373, November 1958.
The cross-section of the conventional wedge shaped anechoic element consists of a square or rectangular base, with two opposite side surfaces of the element tapering to a line junction with one another. The length of the wedge unit. i.e., the distance measured from the line junction to the base, varies according to the low frequency cutoff desired in the chamber. The lower the low frequency cut-off, the longer the wedge unit or overall depth of treatment required to create an anechoic environment. Typically, a quarter wavelength of the desired low frequency cutoff approximates the overall depth of treatment that is required to create an anechoic environment in a test chamber. The depth of treatment is determined by the geometry of the anechoic wedge and the wall of the sound attenuating structure. Various combinations of wedge taper, size, and air gap (if any) between the anechoic element and the supporting wall structure may be required in order to achieve the proper depth of treatment for various low frequency cut-offs.
In addition to the shape of the anechoic wedge unit, both the flow resistance of that wedge unit and the sound absorbing material of which the wedge is constructed are critical to the performance of the wedge-shaped anechoic elements and the chamber that employs same.
There have been a number of changes in the design and construction of units used in the construction of anechoic wedge chambers. These changes have included changes in the sound absorbing materials, along with different protective coverings and support systems. Wedges fabricated from polyether or polyester open cell foams, e.g., polyurethane foams, or melamine, have been used for the wedge material. These have the advantage of light weight.
To increase sound absorbency in anechoic chambers, conventional industry practice has been to mount anechoic wedges having a wedge tip, wedge base, and air space elements in an array of alternating groupings of horizontal and vertical wedges over the entire interior surface of the anechoic chamber. Industry standards dictate that anechoic wedges should achieve greater than 90% sound absorption at the lowest frequency to be measured (the xe2x80x9ccut-off frequencyxe2x80x9d). The shape, dimensions and composition of an anechoic wedge are governed by mathematical equations well known in the art. The size and dimensions of an anechoic chamber depend upon the size of the specimen to be tested and upon the frequency range to be measured. For example, small computer devices and equipment may only require an anechoic chamber the size of a medium-sized room, whereas large construction equipment and jet airplanes may require a chamber as large as an airplane hanger.
The anechoic chamber preferably should be capable of testing specimens at a broad spectrum of cutoff frequencies. The cut-off frequency similarly governs the chamber""s dimensions. To achieve accurate low-frequency measurements, the measuring equipment should be located a sufficient distance from the equipment being tested and from the chamber""s wall. ANSI standards specify that a measuring microphone be located no closer than one meter to the specimen and no closer than xc2xc of the wavelength of the cut-off frequency to the tip of the anechoic wedge. Similarly, the necessary depth of an anechoic wedge is inversely proportional to the specified cutoff frequency. Like the anechoic chamber itself, is the specified cut-off frequency decreases, the wedge depth of a standard anechoic wedge must increase in proportion to the cut-off frequency""s wave length in order to obtain sufficient low frequency sound absorption. Specifically, the wedge depth may be no less than xc2xc of the wavelength of the cut-off frequency. Accordingly, as the cut-off frequency to be measured decreases, the necessary size and dimensions of the anechoic wedges and the anechoic chamber increase. As the specified cut-off frequency decreases, the wavelength of the cut-off frequency and the wedge depth and the size of the anechoic chamber increase proportionately. The increase in wedge depth can often be significant. For example, the industry standard cut-off frequency of 125 hertz would have a wavelength of 2.76 meters and require a wedge depth of 0.7 meters, whereas a lower cut-off frequency of 50 hertz would have a cut-off frequency of approximately 6.9 meters and require a wedge depth of approximately 1.72 meters.
This increase in required wedge depth has presented unique problems for the design of anechoic chambers. Increased wedge depth results in an exponential increase in both the volume and cost of sound absorptive material needed to construct the anechoic wedges.
Similarly, the increased size of the needed anechoic wedge also causes a corresponding increase in the necessary footprint for the anechoic chamber. Unfortunately, due to the low-rigidity of most sound absorptive materials, standard anechoic wedges exceeding a certain wedge depth may bend or break from their mounts under their own weight. At larger sizes, standard anechoic wedges also become extremely cumbersome, difficult to manipulate, and difficult to mount using conventional mounting systems
Also, given the increasing variety of products, industrial machinery, and equipment now being tested, anechoic chambers used to conduct such sound tests are exposed to more rigorous environments. Exposure to such rigorous environments frequently results in damage to and requires the replacement of the delicate sound-absorbing anechoic wedge tips used in such anechoic chambers.
Several techniques have been employed to strengthen and protect the anechoic wedges. One previous technique has been to enshroud the wedge tip and wedge base elements of the anechoic wedge with a wire cloth framework to provide structural support. Unfortunately, the overall size or cost of the wedge is not significantly affected and the direct introduction of such reflective material into the anechoic chamber may result in sound reflections which reduce the accuracy of the measurements. Another attempt at addressing this problem is demonstrated by the sound absorbing unit described in U.S. Pat. No. 5,317,113 in which perforated metal is used to shape, contain and protect the wedge material. Sound absorption may be sacrificed compared with a standard anechoic wedge. According to another previous technique, the wedge tip and wedge base are joined into an integral unit by an exterior housing. To form the air space element of the anechoic wedge, the housing containing the anechoic wedge base and tip is suspended or offset mounted approximately 3xe2x80x3 to 4xe2x80x3 inches away from the anechoic chamber""s inner surface to create the air space important to the function of the anechoic wedge. Several methods are known in the art for mounting the wedge elements in this fashion, including the use of furring strips to offset mount housings containing a configuration of wedge base and wedge tips. Unfortunately, the use of frameworks and offset mounting of the anechoic wedges has turned out to be both costly and maintenance intensive. Typically, damaged wedges cannot be replaced without significant effort and expenses. Often, to replace a single wedge tip, an entire series of wedges must be removed from their mountings.
It is well known that in an anechoic chamber having wedges, the surface of the wedges need not be completely sound absorptive, and, indeed, if such complete absorption was possible, the wedge configuration itself would be unnecessary. Thus, the purpose of the wedges is to partially absorb and partially reflect the acoustic waves. Due to the steep wedge angles, reflected acoustic waves are trapped between adjacent wedges by a process of total internal reflection, such that any portion which is retro-reflected from the recess is highly attenuated. See. Warnaka, U.S. Pat. No. 4,477,505 (see FIG. 2), Pelonis, U.S. Pat. No. 5,141,073 (see FIG. 4), and U.S. Pat. No. 5,317,113, each of which is expressly incorporated herein by reference.
Thus, as long as the surface is only partially acoustically reflective, the elongated wedge will tend to absorb the sonic energy. A number of advantages are apparent from the use of a perforated sheet metal cover over the acoustically absorptive material in the wedge. These covered structures tend to have improved impact resistance, and indeed the acoustically absorptive material placed inside need not be self-supporting. The cover sheet, serves to retain the acoustically absorptive contents, and thus may prevent mineral wool or fiberglass fibers inside from becoming airborne. The perforated surface may also be cleaned, repainted or the like, since the acoustic properties of the surface are non-critical, likewise, under unusual circumstances, a further acoustically absorptive treatment may be applied for enhanced performance. The perforated steel also facilitates fire retardancy, even if the contents are somewhat flammable or contaminated with oil.
Thus, a need has arisen for an efficient anechoic wedge system for anechoic chambers that would employ traditional wedge materials while minimizing the overall size necessary for the wedge and room and providing sufficient protection to the anechoic wedge elements.
Similarly, it would be advantageous to provide a mounting system or method which would protect the anechoic wedge from damage and would permit ease of mounting, repairing and replacing of the anechoic wedges.
The modular anechioic panel system of the illustrative embodiment advantageously provides structural modular anechoic panels for the assembly of wall, roof and/or floor components of an anechoic chamber. Each modular anechoic panel is structurally self supporting and contains the acoustical wedge base and air space elements of an anechoic wedge. In the illustrative embodiment, an acoustically transparent interior shelf and a structural face plate retain the wedge base, air space, and transmission loss material in position within the modular anechoic panel""s structural steel frame. H-joints permit numerous modular anechoic panels to connect to one another to form a shell such that each panel""s face plate becomes a portion of the interior surface of the assembled anechoic chamber. Additionally, a wedge tip compression clip system allows selective mounting of the wedge tips flush to the surface of the face plates.
It has been found that the performance of the anechoic wedges may be improved by providing a configuration of the apertures of a perforated steel sheet facing having a high void ratio, without impairing the substantial mechanical performance of the wedges. In particular, while it is known that the acoustic reflectivity of the wedge surface is not a per se acoustic advantage, the structural integrity of the wedges and the manufacturability thereof is enhanced by providing a void ratio of less than 50%. Thus, traditional practical considerations compelled an acoustically inferior solution.
The present inventor has found, however, that the void ratio of the perforated sheet metal facing sheet may be increased to about 63% while maintaining structural integrity, other advantages of a perforated sheet covering, and manufacturability. Thus, the substantial acoustic reflectivity mandated by the design of Duda, U.S. Pat. No. 5,317,113, is overcome.
In fact, by providing an open mesh arrangement of the cover, acoustic reflectivity is held to low levels, over a broad range of frequencies, thus improving the performance of the wedges. Improvements in acoustic absorptivity, in turn, leads to a reduced need for wedge volume. Thus, for the sale interior volume, smaller external dimensions are required for an anechoic chamber. Further, reduced sheet metal weight leads to educed overall weight, leading to reduced transportation costs and raw material costs. Further, since the wedges must be mounted within the chamber, the mounting hardware is subject to less stress. Maintenance of the chamber, which might involve removal of the wedges, is also facilitated.
It is technical advantage that the incorporation of the anechoic wedge elements with each modular anechoic panel forming the anechoic chamber""s structural shell permits the absorption of sound in an anechoic chamber having a reduced overall room footprint.
In addition, the illustrative embodiment provides a modular design that provides a level of protection to many elements of the acoustic wedge, and is cast efficient to manufacture, assemble, and maintain relative to previous techniques. Moreover, the compression clip system of the illustrative embodiment provides for ease of installation, maintenance, and repair of wedge tips, which are susceptible to exposure and damage. Should a wedge tip become unacceptably soiled or otherwise damaged it can be removed and replaced by hand and at far lessor cost than conventional means.
It is therefore an object of the invention to provide amodular wedge for an anechoic panel, comprising a unitary perforated metal sheet having greater than about 52% void area, formed in a wedge shape having a depth, and having therein an acoustically absorptive material, having an acoustic absorption of at least 90% at a cutoff frequency defined by a corresponding wavelength 4 times the depth. The sheet preferably has less than 80% void area, and more preferably a void area of about 63%.
It is a further object of the invention to provide a system and method for manufacture of anechoic chamber wedges which formed each wedge from a single sheet of perforated metal, bent to shape, and held in conformation by a set of rivets, said rivets passing through regular perforations in the perforated metal sheet.
The wedge is preferably packed with an acoustically absorptive material, such as acoustically absorptive fiberglass, and may be packed in layers to provide further control over absorption characteristics.
It is a further object of the invention to provide a substantially enclosed sound absorbing unit for an anechoic chamber, comprising a substantially flat panel member having a layer of sound absorptive material, and an anechoic wedge member disposed adjacent to said flat panel member, said anechoic wedge member configured having a base and four protruding walls, at least two of which are convergent, formed from a substantially sound transparent sheet having perforations formed therein, said perforations encompassing at least about 52% of the total area of the sheet.